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Chemistry at molecular junctions: Rotation and dissociation of O2 on the Ag(110) surface induced by a scanning tunneling microscope
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Image of FIG. 1.
FIG. 1.

(a) The unit cell of O/Ag(110). O is chemisorbed on the FFH site along the (1 0) direction. The interlayer spacing is defined in terms of the bulk lattice constant, . (b) Top view of the top layer of the Ag(110) supercell showing the three high-symmetry sites studied for chemisorption of O. The green, solid circle shows the FFH site; the maroon, open circle shows the short bridge site; and the purple, solid rectangle shows the long bridge site.

Image of FIG. 2.
FIG. 2.

Top views of DFT-optimized geometries of O chemisorbed on high-symmetry sites of the Ag(110) surface: (a) the FFH site along (001) direction, (b) the FFH site along the (1 0) direction, (c) the long bridge site, and (d) the short bridge site. The blue rectangles denote the boundaries of the unit cells. The Ag atoms in the top surface layer are colored in a darker shade for visual clarity. A few Ag atoms on the edges of the unit cells were displaced outside the cell boundaries during geometry relaxations, and were consequently shifted to the opposite edges due to periodic boundary conditions.

Image of FIG. 3.
FIG. 3.

DFT-computed two-dimensional surface plots of (a) (, θ), (b) (, θ), and (c) (, θ) with and θ shown schematically in (d). The lateral position of the O center-of-mass was held fixed at the FFH site, while the out-of-plane angle and molecule-surface distance were allowed to relax. Results consistently found the molecular axis to be parallel to the surface plane. The dashed arrows show the minimum energy pathways to O dissociations along the (001) and (1 0) directions, and O rotation between the equilibrium geometries along (001) and (1 0) orientations. The small, open circles show the two chemisorption minima, and the small open squares show the transition states along the rotation and dissociation pathways.

Image of FIG. 4.
FIG. 4.

DFT-computed effect of electric field on the zero-point-corrected barrier heights for dissociation and rotation of (a) O{FFH(001)} and (b) O{FFH(1 0)}.

Image of FIG. 5.
FIG. 5.

Morse potential fits to DFT-computed () for dissociation of (a) O{FFH(001} and (c) O{FFH(1 0}, and quadratic fits to DFT-computed μ() for dissociation of (b) O{FFH(001} and (d) O{FFH(1 0}. The DFT results are shown by solid maroon circles and the corresponding fits are shown by maroon curves in all the figures. Panels (a) and (c) show the Morse potential eigenfunctions corresponding to the ground vibrational state (green) and the vibrational state closest to the dissociation barrier (blue). The dashed green and dot-dashed blue lines depict the respective eigenvalues. In panels (b) and (d), the dashed green and dot-dashed red lines denote the bond lengths of chemisorbed O at its equilibrium geometry and dissociation transition state, respectively.

Image of FIG. 6.
FIG. 6.

Rates of dissociation from “dipole”-induced IET as a function of current for O{FFH(001} and O{FFH(1 0}. The solid green line shows the calculated dissociation rate for O{FFH(001)} and the dot-dashed blue line shows the calculated dissociation rate for O{FFH(1 0)}. The red symbols denote the experimental dissociation rates of O on Pt(111) at a bias voltage of 400 mV, as measured by Stipe The dashed red line demarcates the calculated dissociation rate of 5 s for O{FFH(001)} and 0.2 s for O{FFH(1 0)} at a current of 1 nA.

Image of FIG. 7.
FIG. 7.

The PDOS as a function of energy for O chemisorbed on (a) FFH(001) site, and (b) FFH(1 0) site of Ag(110). The plots were calculated by projecting the corresponding total density of states of O/Ag(110) on to the 2s, 2p, 2p, and 2p atomic orbitals of the two O atoms. denotes the Fermi energy of the system. The peaks qualitatively depict the σ, π, and molecular orbitals of O. The 2s components were negligible in the chosen energy range and are therefore omitted in the plots. The Lorentzian fits to the components of the PDOS plots in (a) and (b) are shown in (c) O{FFH(001)} and (d) O{FFH(1 0)}.


Generic image for table
Table I.

DFT-calculated binding properties of O chemisorbed on FFH(001), FFH(1 0), short bridge, and long bridge sites of Ag(110). The subscript “” indicates equilibrium.

Generic image for table
Table II.

Relative populations of O{FFH(001)}:O{FFH(1 0)} at surface temperatures 45 K and 75 K.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Chemistry at molecular junctions: Rotation and dissociation of O2 on the Ag(110) surface induced by a scanning tunneling microscope